One of the most common thermoplastic polyester polymers is polyethylene terephthalate (PET). PET polymer is used extensively in the packaging industry, especially in the production of bottles for carbonated and non-carbonated beverages. PET is widely used because of its excellent combination of clarity, mechanical, and gas barrier properties. Certain applications, however, require higher gas barrier properties than PET exhibits. These include the need for reduced carbon dioxide egress, particularly for smaller carbonated soft drink containers where the surface to volume ratio is large, and reduced oxygen ingress for oxygen sensitive products such as beer, juice, and tea.
There are several examples of additives or other modifications of PET to improve gas barrier properties. These generally fall into two categories: passive gas barrier technology and active gas barrier technology. The contrast between the two is that passive gas barrier technology provides a physical blockage for the movement of small molecules, such as carbon dioxide or oxygen, across the PET container whereas active gas barrier technology includes such things as oxygen-scavenging where the oxygen molecule is actually consumed in a chemical reaction.
A variety of approaches have been used to enhance the passive barrier properties of PET, including, blends with high barrier polymers or additives that decrease the permeability of the resin, incorporation of impermeable fillers, coated and multilayer structures, and copolymerization with comonomers that produce a lower permeability polymer than unmodified PET. Problems with these technologies include decrease in the PET mechanical properties (e.g., decrease in intrinsic viscosity and/or glass transition temperature with additives) and increased capital and operating costs (e.g., additional equipment to apply coatings and/or expense of multi-layered structures).
There is a need to improve the gas barrier properties of PET without significantly impacting the mechanical properties of the PET or significantly increasing costs.